1. Technical Field to Which the Invention Pertains
The present invention relates to a washing liquid and, in particular, it relates to a washing liquid for a semiconductor substrate surface on which a metal wiring material (in particular, Cu) is exposed.
Furthermore, the present invention relates to a washing liquid used in the removal of micro particles and metal impurities adhering to the surface of a semiconductor substrate after chemical-mechanical polishing (hereinafter called CMP), particularly in a semiconductor production process.
2. Prior Art
Accompanying the increasing integration of ICs, there is a demand for strict contamination control since trace amounts of particles and metal impurities greatly influence the performance and yield of a device. That is, strict control of particles and metal impurities adhering to the surface of a substrate is required, and various types of washing liquids are therefore used in each of the semiconductor production processes.
With regard to washing liquids generally used for semiconductor substrates, there are sulfuric acid-hydrogen peroxide, ammonia-hydrogen peroxide-water (SC-1), hydrochloric acid-hydrogen peroxide-water (SC-2), dilute hydrofluoric acid, etc., and the washing liquids are used singly or in combination according to the intended purpose. In recent years, the CMP technique has been introduced into semiconductor production processes such as planarization of an insulating film, planarization of a via-hole, and damascene wiring. In general, CMP is a technique in which a film is planarized by pressing a wafer against a cloth called a buff and rotating it while supplying a slurry, which is a mixture of abrasive particles and a chemical agent, so that an interlayer insulating film or a metal material is polished by a combination of chemical and physical actions. Because of this, the CMP-treated substrate is contaminated with particles and metal impurities including alumina particles and silica particles, which are used in the abrasive particles in large amounts. It is therefore necessary to employ cleaning to completely remove these contaminants prior to the process that follows. As a post-CMP washing liquid, an aqueous solution of an alkali such as ammonia is conventionally used for removing particles. For removing metal contaminants, techniques using an aqueous solution of an organic acid and a complexing agent have been proposed in JP, A, 10-72594 and JP, A, 11-131093. As a technique for simultaneously removing metal contaminants and particulate contaminants, an aqueous washing solution in which an organic acid and a surfactant are combined has been proposed in JP, A, 2001-7071.
When the use of CMP was limited to the planarization of interlayer insulating films and via-holes, since a material having poor chemical resistance was not exposed on the surface of a substrate, washing with an aqueous solution of ammonium fluoride or the above-mentioned aqueous solution of an organic acid could be employed. However, the damascene wiring technique has been introduced as a technique for forming Cu wiring necessary for increasing the response speed of semiconductor devices, and at the same time there have been attempts to use, as the interlayer insulating film, a low permittivity organic film such as an aromatic aryl polymer, a siloxane film such as MSQ (Methyl Silsesquioxane) or HSQ (Hydrogen Silsesquioxane), a porous silica film, etc. Since these materials do not have sufficient chemical strength, use of the above-mentioned alkaline substance or fluoride as a washing liquid is limited.
On the other hand, it can be expected that corrosion of a low-permittivity insulating film or Cu by the above-mentioned composition employing an organic acid will be low and this composition is most preferable. However, there are new problems in the process for forming Cu wiring. One thereof is that even the use of an organic acid causes etching of the surface of copper to a slight degree. Another thereof is that micro corrosion defects of copper are observed during the damascene process for embedding copper wiring. In this case, copper wiring is formed by coating trenches formed in an insulating film with a barrier metal such as Ta or TaN, then embedding copper, and polishing and removing, by CMP, blanket copper formed on the surface. It is therefore necessary to remove slurry-derived polishing particles, dust produced by polishing, and metal impurities adhering to the surface of the substrate after CMP. However, when the embedded copper wiring is exposed, contact thereof with either an acidic or alkaline chemical liquid can generate wedge-shaped micro corrosion of copper at the interface between the copper and the barrier metal such as Ta or TaN, thus degrading the reliability of a device. Accompanying further miniaturization of the device, during the Cu wiring formation process, such an interface between the copper wiring and the barrier metal might be exposed by the use of a washing liquid, and these corrosion defects become more apparent, thus causing deterioration in the performance and quality of the electrical characteristics. Such corrosion is called a side slit.
It is known that side slits can be caused by an aqueous solution of an organic acid such as oxalic acid, malonic acid, or citric acid, which are conventionally said to be non-corrosive.
Yet another problem relates to corrosion of a wiring material formed from a copper alloy, to which a different type of metal has been added. The copper alloy has local areas where the different type of metal is in contact with the copper, and these areas have the same problem as that of the above-mentioned structure where the barrier metal such as Ta is in contact with copper. That is, if such copper alloy wiring is exposed, even when a conventional aqueous solution of an organic acid is used, contact with the organic acid can easily cause corrosion at the interface between the copper and the different type of metal, and there is a possibility of surface roughness, side slits, and pit-shaped corrosion defects being caused on the copper alloy.
JP, A, 2002-69495 discloses a washing liquid that contains a reducing agent and has a pH of 3 to 12 as a washing liquid that can remove micro particles and metal impurities without corroding a wiring material such as copper or tungsten. However, the higher the pH, the easier it is for side slits to occur and for the reducing agent to decompose. Furthermore, there is a problem that an ammonium compound might cause micro corrosion of copper.
As hereinbefore described, there are various types of washing liquids suitable for conventional wiring materials and interlayer insulating films, but under the current circumstances there is no washing liquid that can simultaneously satisfy the above-mentioned needs of a semiconductor substrate that, for example, has copper wiring exposed and has a structure in which copper is in contact with a different type of metal.